Glycogen stores may fluctuate around rather low or rather high average levels, as part of the metabolic adaptation permitting carbohydrate oxidation to be commensurate with the diets corbohydrate contents. The hypothesis which will be tested is that an expansion or reduction of the adipose tissue mass also represents and adaptation, influencing free-fatty acid levels until fat oxidation becomes commensurate, percentage-wise, with a particular diet's fat content. Long-term weight maintenance would then be achieved thanks to the ubiquitous ability for maintaining carbohydrate balance by mutual adjustment of food intake to carbohydrate oxidation and vice-versa. But whereas changes in the glycogen stores hardly affect body weight;, alterations of the adipose tissue mass can lead to obesity. Herein could be found a simple explanation for the high incidence of obesity in affluent societies consuming "mixed" diets with a relatively high-fat content. The relationship between the fat content of the diet and the steady-state adipose tissue mass will be studied by carcass analysis of mice adapted to diets with a fixed protein, but variable carbohydrate and fat contents (12 to 83 percent calories as fat). The experiments will be designed to reveal whether this relationship is progressive and reversible, which would confirm the adaptive nature of diet-induced changes in adiposity. Some animals will be kept in hermetically sealed chambers, vented once every 24 hours, to measure their O2 consumption and CO2 production. In conjunction with measurements of nutrient intake (corrected for spillage and incomplete absorption) this will allow to calculate daily changes in the animals' carbohydrate and fat contents. Such data will allow to determine how accurately the carbohydrate, fat and energy balances are maintained under ad libitum food intake conditions, and to examine whether and how food intake may be altered in response to gains, or losses in the animal's glycogen and/or fat reserves. The general goal of the proposed investigations is to demonstrate the impact of the dietary fat content on steady-state body composition. Ultimately this may allow to develop metabolic arguments (in addition to those based on caloric density) suggesting that a reduction in the diets'fat content may facilitate weight maintenance at a lower level of adiposity.